One. First he'll read as many books about lightbulbs as his local library has available, if they aren't sufficient he'll use the library computer to order more, then he'll double-check how to change one on the internet, preferably on Wikipedia. After correcting any spelling and grammar mistakes on Wikipedia's 'lightbulb' entry, he'll change the bulb. Having done that, he'll read any other lightbulb books he can get, continue to research, and will start collecting lightbulbs, while talking about them to anyone who stands still for long enough.
Alternative answer: One, but if there are more aspies than that in the room, they'll stand about awkwardly for a while before holding a committee about who should change it.
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shadexiii says, 'Don't drink the kool-aid.'